Succinate protection against oxygen toxicity in rats was investigated at oxygen pressures of 5, 7, 9, and 11 atmospheres absolute (atm abs), and compared with protection given by dextrose and malate. Times to convulsions were used for comparison. Dextrose and malate gave no protection. Succinate provided relatively long-term protection at 5 and 7 atm abs oxygen exposures, but showed decreasing protection with increasing pressure. Succinate provided no protection at oxygen pressures of 12 atm abs or higher. A 3:1 sodium succinate:ammonium succinate mixture (0.4M, pH 7.4) solved the problem of alkalosis previously encountered with a sodium succinate solution. The infusion of the 3:1 mixture (6 millimols/kg/hr) in dogs exposed to 5 atm abs oxygen delayed convulsion 5 to 11 times longer than in controls. Intracellular localization and thin layer chromatography studies with succinate-2, 314C showed that succinate readily penetrated the blood brain barrier.